Biomass response and changes in composition of ephemeral macroalgal assemblages along an experimental gradient of nutrient enrichment

Most conceptual marine eutrophication models predict that ephemeral and epiphytic macroalgae will become substantially more abundant with increasing nutrient richness. This expectation is based on the fact that most of these fast-growing algae possess high requirements for inorganic N and P and, therefore, suffer from nutrient limitation under nutrient poor conditions. We tested the hypothesis that nutrient enrichment will stimulate the abundance of ephemeral macroalgae by studying the abundance and species composition of these algae on several types of substrata along an artificial nutrient gradient. The total biomass of ephemeral macroalgae differed considerably among different types of substrata. The total ephemeral load was significantly higher on consolidated substrata than on large perennial algae, but we were unable to detect any systematic increase in the abundance of these ephemerals with increasing nutrient richness. We found, however, indications that the composition of the ephemeral assemblage changed with nutrient richness. Hence, corticated filamentous algae (mostly red algae) were more abundant at low nutrient richness while thin foliose algae (mainly green species) tended to become more abundant with increasing nutrient richness in most of the surveyed assemblages.